Typical aircraft turbofan jet engines include an engine core, a nacelle that surrounds the engine core, and a fan that draws in a flow of air that is split into bypass airflow and engine core airflow. The nacelle provides a bypass duct that surrounds the engine core. The bypass airflow is transported through the bypass duct. The nacelle is configured to promote laminar flow of air through the bypass duct. The engine core includes a multi-stage compressor to compress the engine core airflow, a combustor to add thermal energy to the compressed engine core airflow, and a turbine section downstream of the combustor to produce mechanical power from the engine core airflow. The typical turbine section has two and sometimes three turbine stages. The turbine stages are used to drive the compressor and the fan. After exiting from the turbine section; the engine core airflow exits through an exhaust nozzle at the aft end of the engine.
In a turbofan engine, the fan typically produces a majority of the thrust produced by the engine. The bypass airflow can be used to produce reverse thrust typically used during landing. Thrust reversers mounted in the nacelle selectively reverse the direction of the bypass airflow to generate reverse thrust. During normal engine operation, the bypass airflow may or may not be mixed with the exhausted engine core airflow prior to exiting the engine assembly.
Several turbofan engine parameters have a significant impact upon engine performance. Bypass ratio (BPR) is the ratio of the bypass airflow rate to the engine core airflow rate. A high BPR engine (e.g., BPR of 5 or more) typically has better specific fuel consumption (SFC) and is typically quieter than a low BPR engine of equal thrust. In general, a higher BPR results in lower average exhaust velocities and less jet noise at a specific thrust. A turbofan engine's performance is also affected by the engine's fan pressure ratio (FPR). FPR is the ratio of the air pressure at the engine's fan nozzle exit to the pressure of the air entering the fan. A lower FPR results in lower exhaust velocity and higher propulsive efficiency. Reducing an engine's FPR can reach a practical limit, however, as a low FPR may not generate sufficient thrust and may cause engine fan stall, blade flutter, and/or compressor surge under certain operating conditions.
One approach for optimizing the performance of an engine over various flight conditions involves varying the fan nozzle exit area. By selectively varying the fan nozzle's exit area, an engine's bypass flow characteristics can be adjusted to better match a particular flight condition, for example, by optimizing the FPR relative to the particular thrust level being employed. For example, a variable area fan nozzle (VAFN) assembly that forms a rear outer portion of the bypass duct can include an airfoil that is moved aft into a VAFN flow position so as to open an additional bypass flow that exits the nacelle forward of the VAFN assembly. That is, an opening is created between the translatable sleeve and the VAFN airfoil, such that an airflow in the bypass duct is split into a flow that remains in the bypass duct and moves past the airfoil, and a flow that exits the bypass duct through the VAFN opening and over an outer surface of the airfoil.
Turbofan engines typically include a thrust reverser operation, in which the translatable sleeve of the VAFN assembly is moved to expose a cascade array opening, and blocker doors are deployed from in front of the airfoil into the bypass duct. In this thrust reverse position, the blocker doors redirect the airflow in the bypass duct to exit out the cascade array. The VAFN airfoil is typically much too thin and fragile to sustain the airflow loads necessary to divert the bypass airflow to the cascade array. Therefore, the blocker doors are typically deployed from other structures so the blocker doors extend into the bypass duct flow. The VAFN assembly can be selectively positioned anywhere between a stowed position in which no additional bypass exit is formed, a VAFN flow position in which the additional bypass flow exits the bypass duct through the VAFN opening, and a fully deployed position in which the additional bypass exit is open to a maximum extent, such as for thrust reverse operation.
Improving the aerodynamic flow through the bypass duct and around the engine nacelle is a matter of continuous concern for aircraft design. Therefore, it is desirable to locate the VAFN assembly so as to improve aerodynamic flow Improved VAFN assemblies are desired.